In the manufacture of media for magnetic recording, considerable effort is directed at obtaining very smooth, and in the case of rigid media, very flat surfaces in order to minimize wear, and to maximize recording performance and reliability. Regarding rigid media, it is especially important to remove, or at least substantially limit the height of asperities, or projections, above the surface, since such asperities can give rise, directly or indirectly, through high-velocity impact, to catastrophic failure of the read/write head/disk interface. The acceptable height of any such asperities is, of course, related to the intended spacing between the head and the surface of a disk--the so-called flying height--during normal operation. Thus, as flying height is lowered in order to improve performance, the tolerable height of asperities decreases, and the task of creating such smooth surfaces becomes not only extremely important, but also very difficult and costly.
In the preparation of thin-film rigid media, conventional practice has been to roughen or "texturize" the surface of the media substrate prior to deposition of the magnetic storage layer(s) in order to reduce the eventual area of contact between the head and media surface at zero or low velocities, thereby to reduce "stiction", a tendency for a head to stick to the media surface. Such texturing processes occasionally result in the creation of asperities which protrude well above the general surface. As a consequence, subsequent deposition of the magnetic storage and protective overcoat layers, whether accomplished by sputtering or other processes, results in a conformal surface, thereby preserving or even accentuating the asperities. Asperities may also be created in the process of depositing these layers.
While the number and magnitude of such asperities can be reduced by appropriate process, materials and environmental controls, it is all but impossible to eliminate them completely--a task which is necessary to guarantee the most reliable operation. Consequently, various means and approaches have been used to remove or reduce the height of these asperities, including "kiss buffing" and/or burnishing the final media surface. The former technique employs an abrasive tape placed in very light contact with a spinning media disk. The latter is accomplished by flying a specially designed burnishing head above the media surface, at a head/media separation well below that at which a read/write head would normally fly. Asperities are thereby sheared off or plastically deformed. Since a conventional burnishing head is typically comparable in size and mass to a conventional read/write head, and further is subject to a comparable load, there is a risk that the process of burnishing may itself lead to a head crash, or damage to the media surface. Thus, the challenge is for the conventional burnishing head to fly high enough to avoid damaging contact with the media surface yet low enough to intercept and remove the asperities. A burnishing head accomplishes the delicate task just described when subsequent glide-height tests, employing a low-flying head fitted with a piezoelectric transducer, fails to indicate contact between the head and disk, in compliance with specified criteria.
As should be apparent from what has just been described, the risk of damaging a media surface in a burnishing operation increases rapidly and dramatically as the desired flying height (for ultimate intended performance) decreases.
The present invention recognizes the important need to eliminate the possibility of head crash during a burnishing operation in order to make substantial further progress in minimizing asperities and in enabling reliable operation, at very low flying heights, and ultimately in continuous sliding contact, when the surface of the finally burnished disk is used in conjunction with a read/write head. Catastrophic (crash caused) failure of the head/media interface results when, for a given relative velocity, the local pressure, i.e., the force per unit contact area, becomes very large, thereby giving rise to extreme local temperature, and to complex and irreversible physical and chemical reactions. The key, accordingly, is to limit the maximum possible pressure in any region of contact, resulting from applied load and inertial forces, to a level well below the threshold of failure.
Experience indicates that the area of contact between a typical slider and the surface of a disk is ordinarily a very small fraction of the footprint of the slider, because contact occurs at a corner, or along an edge, or with a particle contaminant. From this observation one can conclude that, to a first approximation, the area of contact is relatively independent of the size and mass of the slider. However, a relatively large slider with a relatively large mass requires a relatively large applied force for reasons of dynamic response. Thus, the maximum local contact pressure experienced by a very small, low-mass slider may be orders of magnitude lower than that for conventional sliders, with the result that catastrophic failure is extremely unlikely. Numerous tests have validated this logic. Of course, it is essential that the materials used in the construction of both the slider and the protective media overcoat, as well as in any lubricant employed, be chosen and processed optimally in order to provide maximum freedom from head crash.
On the basis of this reasoning, the present invention proposes and describes an integrated, unitary, micro-burnishing flexible (or flex) head structure, having very low mass, and designed to operate in continuous sliding contact at relatively high velocities, and capable of rapid dynamic response to media run-out. The latter property ensures that the cutting edge of the burnishing head remains in contact with the media surface, and that any asperity protruding from that surface will be sheared off.
In its most simplified form, the burnishing head structure of the present invention takes the form of an elongate flexure body, adjacent one end of which there is joined a low-mass abrader head formed of a suitable high-hardness material which exists either as a single crystal, or as a low-porosity, fine-grain-structure polycrystalline material. What might be thought of as the leading edge of the abrader head may, as will be explained below, occupy various angles and dispositions as the same is presented to the surface of a disk to be burnished, with this edge designed to be self-sharpening over a relatively long period of usage.
In a somewhat more involved embodiment of the invention, one that appears to offer particular utility in the mass manufacturing of disks, what is proposed is a comb-like array structure including a plurality of flexure bodies, capable of independent flexure, each carrying an abrader head at one end and joined adjacent their opposite ends to a common joinder structure.
A head structure constructed according to the invention is designed to operate with an effective load of about 20- to about 100-mg.
Details of these proposed constructions will now become more fully apparent as the description which follows is read in conjunction with the accompanying drawings.